Environmental Engineering Reference
In-Depth Information
Figure 5.
Working model for U(VI) reduction pathway in S. oneidensis.
3. MECHANISM OF TECHNETIUM REDUCTION
Molecular studies on the ability of microorganisms to reduce Tc(VII) have
been focused in E.coli , however the ability to reduce Tc(VII) has been recently
established in Shewanella putrefaciens CN32, Shewanella oneidensis MR-1 and
Shewanella putrefaciens 200R [40, 50, 72, 91]. Tc(VII) is also reduced under
acidic conditions by Thiobacillus thiooxidans [50], under alkaline conditions
by Halomonas strain Mono [33] and at high temperature by Pyrobaculum
islandicum [32]. Reduction of soluble Tc(VII) results in formation of Tc(IV)
which precipitates as insoluble TcO 2 and is immobilized in the environment.
Immobilization also occurs via formation of strong surface complexes with
hydroxylated surface sites on Al and Fe oxides and clays in the absence of
aqueous complexing agents [20, 27, 55, 75, 91].
3.1 The Role of Hydrogenases in Tc(VII) Reduction
E. coli possesses four hydrogenases, designated as hydrogenases 1-4. Hydro-
genases 1 and 2 share little homology to hydrogenases 3 and 4. Hydrogenases
3 and 4 share high homology to each other and are both expressed as part
of the formate-hydrogen lyase complex [4]. The Tc(VII) reductase in E.coli
has been identified as the Ni-Fe hydrogenase 3 component of the formate-
hydrogen lyase complex [40]. Hydrogenase expression is determined by pH:
hydrogenase-4 (encoded by the hyf operon) is expressed under alkaline con-
ditions while hydrogenase-3 (encoded by the hyc operon) is expressed under
acidic conditions [4]. The formate hydrogen-lyase complex in E.coli is com-
posed of formate dehydrogenase plus multiple components of the respective
hydrogenase encoding operons (i.e., hyf and hyp operons). The bi-directional
nature of hydrogenase-3 (HycE) enables both the production of H 2 during
formate oxidation and the direct oxidation of H 2 .
S. oneidensis MR-1 does not possess a formate-hydrogen lyase complex and
possesses only two hydrogenases, neither of which share significant homology
to hydrogenase 3 or 4 of E.coli . The first S. oneidensis MR-1 hydrogenase
(Locus SO2098; HyaB) displays high homology to the IM-bound Ni-Fe hydro-
genase HydB of Wolinella succinogenes , while the second S. oneidensis MR-1
hydrogenase (Locus SO3920; HydA) displays high homology to the (putative)
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